[clang] [llvm] Reapply "[AArch64][SVE] Improve fixed-length addressing modes. (#130263)" (PR #130625)

[Paul Walker via cfe-commits]

paulwalker-arm wrote:

Sorry for the delay and thanks for the investigation @rj-jesus. This is sooooo not intentional behaviour.  VLS based auto vectorisation was implemented before the VLS ACLE extensions and by that time it's likely fixed length calls to `llvm.vscale()` were constant folded away and so never made it to code generation, hence why we've survived this long without being bitten.

I think the problem sits in `getMemVTFromNode()` which is implemented too literally when considering fixed length vectors.  The function should be return the memory footprint of the operation and for that only the element type of the MemVT matters because that is what governs when extension/truncation must be considered.

I've quickly tested the following, which corrects the behaviour for your test case but I've not investigated what other nodes need to be handled so as not to hit the unreachable.
```
 static EVT getMemVTFromNode(LLVMContext &Ctx, SDNode *Root) {
-  if (isa<MemSDNode>(Root))
-    return cast<MemSDNode>(Root)->getMemoryVT();
+  if (isa<MemSDNode>(Root)) {
+    EVT MemVT = cast<MemSDNode>(Root)->getMemoryVT();
+
+    EVT DataVT;
+    if (auto *Load = dyn_cast<LoadSDNode>(Root))
+      DataVT = Load->getValueType(0);
+    else if (auto *Load = dyn_cast<MaskedLoadSDNode>(Root))
+      DataVT = Load->getValueType(0);
+    else if (auto *Store = dyn_cast<StoreSDNode>(Root))
+      DataVT = Store->getValue().getValueType();
+    else if (auto *Store = dyn_cast<MaskedStoreSDNode>(Root))
+      DataVT = Store->getValue().getValueType();
+    else
+      llvm_unreachable("Unexpected MemSDNode!");
+
+    return DataVT.changeVectorElementType(MemVT.getVectorElementType());
+  }
```

Do you mind running with such an approach for your PR? If not, I'm happy to finish it off and push a PR for yours to build upon.

https://github.com/llvm/llvm-project/pull/130625